The present invention relates to an improvement in the resistance welding of galvanized, i.e. zinc or zinc alloy coated, steel parts or sheets which is achieved by coating either the galvanized surface to be welded or the welding electrode with a resin binder containing a metal phosphide pigment, and preferably a ferrophosphorus pigment. The welding improvements realized by practicing the present invention are improved weldability lobes and dynamic resistance curves for better welding control for resistance welding systems as well as increased electrode life.
The use of galvanized steel sheets in the automotive industry has become increasingly popular in recent years due to the increase in concern for corrosion protection for automobile body panels. Corrosion problems are particularly severe in environments where salt is used for preventing the icing of snow on highway roads. Although efforts have been made to enhance the corrosion-resistance of steel sheets, such as by using various chemical conversion treatments and paint coatings, the corrosion protection method of choice currently is galvanized steel, with the galvanized coating formed by either hot-dipping or electrodeposition.
For zinc or zinc alloy coated sheet steels to successfully substitute for uncoated sheet steels, they must exhibit acceptable formability and weldability characteristics. As a general rule, coated steels have not demonstrated properties as good as their uncoated counterparts. Users of these products are continually looking for new coated sheet steels which provide the advantages of a coated steel, but have weldability and formability characteristics similar to uncoated steels.
The most common method of joining steel sheets (particularly in the automotive and appliance industries) is resistance spot welding. Resistance spot welding is ideally suited for joining thin sheet materials and is well adapted to mass production industries. In addition, operating costs for this process are relatively low. Resistance spot welding has been used with uncoated steels quite successfully since the 1930's.
Resistance spot welding is used to form joints between two materials. The process uses a set of electrodes to apply pressure to the weld area, to maintain the components in position, and to pass current through the weld. As the current flows, joule heating of the substrate occurs. Due in part to the cooling effects of the electrodes, a molten nugget eventually develops at the weld centerline or faying surface. On cooling, this nugget resolidifies and effects a joining between the two materials.
As mentioned, resistance spot welding of uncoated steels has historically been quite successful. However, the resistance spot weldability of coated sheet steels has not been as successful. The problems can be best seen by reference to some typical measures of spot weldability.
The weldability lobe is defined as the range of welding conditions (weld current and weld time) over which weld nuggets of an adequate size can be formed. This, in effect, defines a "window" of acceptable welding conditions. When practical, weld nugget sizes during lobe testing are estimated with a destructive test known as the peel test. This test consists of welding two 11/4-inch by 4-inch samples at two points, and destructively pulling apart the second of the welds. The weld nugget will usually adhere to one of the two sheets as a weld "button", and the size of this weld button can be measured with a set of calipers. The weld button size is usually considered a good measure of the nugget size. The limits of the weldability lobe are defined by the welding conditions which produce a minimum weld size on one side, and expulsion on the other (expulsion occurs when liquid metal is expelled from the weld during welding). A line representing a nominal button size (part way between the minimum and expulsion) is also often included.
The weldability lobes are characterized by lobe position, lobe width and the position of the nominal button line. See, generally, D. W. Dickinson, Welding in the Automotive Industry, Report SG 8-15 of the Committee of Sheet Steel Producers, the American Iron and Steel Institute. The lobe position is defined as the average welding current of the lobe. Though lobe position is not considered to be a critical weldability parameter, higher welding currents do result in higher energy costs, as well as a decrease in electrode life. More significant is the width of the weldability lobe defined as the difference in welding currents between minimum button and expulsion at a particular welding time. This is a measure of a materials' "flexibility" during spot welding. The position of the nominal button line, although considered of lesser importance, is also a measure of a materials' flexibility during spot welding. A central position for this line indicates a button size with adequate current range to both higher and lower currents.
Dynamic resistance is used as a measure of weld quality and is defined as the resistance of the weld across the electrodes (as a function of time) during welding. The dynamic resistance has been correlated to weld development in uncoated steels, and successfully used as an input signal for feedback control. Unfortunately, the results for zinc or zinc alloy coated steels have not been as good. In particular, feedback systems have been largely unsuccessful in controlling weld development in such coated steels which exhibit a featureless resistivity trace or curve. The dynamic resistance trace for uncoated steel, in contrast, exhibits a characteristic "beta peak", followed by a resistance drop. It is the presence of this "beta peak" which makes resistive feedback control possible. See Dickinson, supra.
When resistance welding uncoated steels, a single set of copper welding electrodes can be expected to make approximately 50,000 welds. When welding galvanized steels, however, the electrode life is reduced to about 1000-2000 welds or less. Since the production line must be stopped each time an electrode is replaced, at a considerable expense to the user, the relatively limited electrode life for galvanized steels represents a significant economic disadvantage.
The use of ferrophosphorus pigment for both improved corrosion protection and weldability has been suggested in the prior art. For instance, U.S. Pat. No. 3,884,705, issued May 20, 1975 and U.S. Pat. No. 4,119,763, issued Oct. 10, 1978, both disclose the use of coatings containing ferrophosphorus and zinc pigments, and a non-metallic corrosion inhibitor such as zinc chromate, as a replacement for zinc-rich coatings. These coatings also contain a non-metallic corrosion inhibitor such as zinc chromate. As contemplated in these patents, the ferrophosphorus pigment-containing coating is applied to bare steel panels rather than to galvanized sheets. The ferrophosphorus pigment used in such applications is commercially available from the Occidental Chemical Corporation under the trademark Ferrophos.RTM. pigment.
A ferrophosphorus pigment dispersed in a resin to bind adjacent steel plates to form a vibration-damping composite suitable for resistance welding is disclosed in Japanese Patent Application No. 61-41540, published on Feb. 27, 1986.
The use of a coating comprising a resin, ferrophosphorus powder and mica powder applied to a steel sheet having a layer of fused aluminum or an aluminum/zinc alloy is disclosed in Japanese Patent Application No. 591456884, published Aug. 22, 1984. The steel sheet described in this reference can be subjected to chemical conversion, and is further described as having excellent weldability, processability and corrosion and heat resistance.
The use of an iron layer containing less than about 0.5 weight percent phosphorus applied to a zinc/iron or zinc/nickel alloy electroplated steel sheet for improved surface properties is described by Honjo et al. in Internal Journal of Materials and Product Technology, Vol. 1, No. 1, pp. 83-114 (1986).
It will be appreciated by those skilled in the art that a continuing need exists for steel sheets which possess the durability and corrosion resistance of galvanized sheets but also possess the weldability advantages of bare steel.